So for those who don’t know, umbrakinesis is the ability to manipulate darkness and shadows, absences of light that commonly appear as pitch black amorphous gas like/liquid like substance capable of many things, but of course this isn’t how darkness or shadows act at all, and if anything, shadows shouldn’t even be a dangerous ability, since they technically are just nothingness, you can’t control nothingness with umbrakinesis, so what exactly is shadow based abilities actually controlling?
If we take a look at this power’s obvious counterpart, photokinesis, light manipulation, this ability allows the user to control what essentially is radiation, which is a physical and tangible thing found in the universe capable of being a dangerous threat due to its speed and burning effects. This ability manipulates a real physical thing, and isn’t some weird abstract concept that uses bullshit like “oooh alternate dimensions give me my powers” to make it work. (shadow users commonly hide behind this trope to justify their confusing abilities.)
However I want to propose a possible answer as to what exactly umbrakinesis as a general ability is actually manipulating, of course, everyone’s fictional universe is different, and the laws of physics and reality could work different for each one, so take what I say with a heavy grain on salt. For example, shadow magic in AO (the 
) is not really a shadow, but magic energy imitating the phenomenon found in nature.
Anyways, my hypothesis/headcanon is that umbrakinesis could be controlling dark matter/dark energy, both of which are hypothetical things to exist in the universe that play key roles in the universe and expanding it/keeping it together. In my head, the reasoning is because it is hypothesised that both of these mysterious fundamental forces in the universe do not emit/reflect light, and hence why it would make sense for it to be completely black in appearance. Now as for it’s offensive capabilities debunked, this is where we hit a slippery slope, literally almost nothing is known for how dark matter / energy work, and that obviously means we haven’t done much testing on it or see how it interacts with physical objects, however, this explanation for what umbrakinesis actually is works pretty well, afterall, pretty much every superpower hits a slippery slope somewhere down the line.
TL;DR: shadow control in general media is most likely a form of dark matter or energy, a fundamental universal force.